The present invention relates generally to apparatus used to dispense a plurality of liquids through a single orifice, and more particularly, to the foam dispensing gun and heating apparatus used to heat the foam gun block through which the chemical reactants required to form polyurethane foams are dispensed.
Polyurethane foams are formed by the reaction of an isocyanate component and hydroxyl-bearing compounds. When mixed together in the presence of a catalyst, a surfactant, and a blowing agent, these chemicals react to form cellular cross-linked polymer chains, more commonly known as a polyurethane foam. Each of these components of the plural component material, by itself, is generally stable. Thus, each component will not cure or cross-link for extended periods of time, often as long as several months, if they are properly stored.
When the isocyanate component and the polyol component are mixed together in proper concentrations and at the proper temperature, an exothermic chemical reaction of the isocyanate and the polyol occurs. The polyol component is a preformulated compound that includes a surfactant, a catalyst and a blowing agent. The resulting exothermic reaction causes a continuing expansion of the mixed components that is evidence of the polymerization and manifests itself as a foam which cross-links and cures. The cross-linking and curing usually is substantially completed in a matter of seconds.
Polyurethane foam dispensers are well known and have achieved a high level of usage in factories where components must be adhesively lined with an insulating foam or where products must be packaged and protected from damage during shipment. In an effort to improve the efficiency of the operation of these foam dispensers and the quality of the product foam, attention has been focused on the factors which enhance the mixing of the plural components in a mixing chamber within the dispensing apparatus. One of the factors which can affect the quality of the mixed foam is the temperature of the components as they are fed into the mixing chamber. Keeping the plural component streams heated to between about 100.degree. to about 125.degree. Fahrenheit promotes thorough mixing and reaction of the plural components.
In an attempt to better control the temperature of each of the plural components delivered to the mixing chamber of a foam dispenser, various approaches have been taken to heat the chemicals prior to their entry into the mixing chamber. Storage tanks holding the separate isocyanate and polyol components have been heated. The feed lines from the storage tanks to the foam dispensing apparatus have also been heated by a variety of means. One method has employed a resistance heater positioned along the flow lines through which the fluids must pass at a specific point between the storage tanks and the foam dispenser. Another and more common approach, has been to use electric resistant heater coils inside each of the hoses which deliver the polyol and isocyanate components to continuously heat the plural components as they travel from the storage container to the dispensing apparatus.
However, none of these prior approaches deal with the problem of the initial few "cold" shots of foam that come from a foam dispensing apparatus that has been left overnight or left unused for a prolonged period of time so that the temperature of the foam dispenser is closer to the temperature of the ambient air than the heated temperature of the plural components being sent through from the feed hoses for mixing in the foam dispenser. The temperature of the foam dispensing apparatus, which is normally metallic, does not reach the same temperature of the heated plural components until several shots of mixed foam have been produced. Until this conductive heating of the gun or foam dispensing block has occurred by the passage of the heated components therethrough, inferior mixing of the isocyanate and polyol components occurs and an inferior quality foam is produced. Normally, these initial few shots of foam are discarded or wasted until the dispensing apparatus reaches proper operating temperature. This problem can be more acute where the dispensing apparatus is operated in a colder climate or in an unheated environment.
This problem is solved in the design of the present invention by providing in the foam dispensing apparatus that is used to dispense plural component polyurethane foam heating means within the foam dispensing gun block sufficient to heat the material of the foam dispensing gun block adjacent each component infeed.